Since the 1960s, the photosynthetic productivity of leaves in the presence of regulated excitation dissipation has been assessed using the lake or puddle models, which assume that excitations travel either infinitely far or very locally regardless of the extent of quenching. Here, by rigorously coarse- graining a nanoscopic model of photosystem II light harvesting, we show that excitation dissipation at any proposed pigment site effectively decreases the excitation diffusion length at the scale of the thylakoid membrane. The resulting decrease in the pool of excitations available to reaction centers for photochemistry can be accounted for by a simple correction to the lake model. Looking forward, the excitation diffusion length provides a useful conceptual tool for connecting excitation dissipation at the molecular scale with higher order effects on crop yield.